Confocal imaging equipment in particular for endoscope

ABSTRACT

Equipment includes an image guide ( 1 ) consisting of flexible optical fibers with: on the proximal end side: a source ( 2 ), angular scanning elements ( 3 ), injection elements ( 4 ) in one of the fibers, elements for splitting ( 5 ) the illuminating beam and the backscattered signal, elements for spatial filtering ( 6 ), elements for detecting ( 7 ) the signal, electronic elements ( 8 ) for controlling, analyzing and digital processing of the detected signal and display; and on the distal end side: an optical head ( 9 ) for focusing the illuminating beam exiting from the illuminated fiber. The scanning elements include a resonant line mirror (M 1 ) and a galvanometric field mirror (M 2 ) with a variable frequency and two afocal optical systems adapted to conjugate the two mirrors (M 1 , M 2 ) firstly in the field mirror (M 2 ) and the injection elements ( 4 ) in the image guide in a second step.

FIELD OF THE INVENTION

The present invention relates to a confocal imaging equipment inparticular for an endoscope and of the type using a flexible opticalfibre bundle. The confocal character resides in the use of the same pathfor illumination and detection, and in the spatial filtering of thesignal collected from the subsurface analysis plane.

The fields of application of the invention are in-vivo biological tissueanalyses, on humans or animals, external, for example in the field ofdermatology, or internal and accessible using the instrument channel ofan endoscope into which the flexible optical fibre bundle can beintroduced, and also the ex-vivo analysis of tissue samples frombiopsies, and the in-vitro analysis of culture in cell biology. Moreoveralso, the device can serve for the analysis of the interior of amanufactured device.

BACKGROUND OF THE INVENTION

At present the medical fields of gastroenterology, respirology,gynaecology, urology, otorhinolaryngology, dermatology, ophthalmology,cardiology and neurology are concerned.

The use of a flexible optical fibre bundle with a small diameter(several hundred microns) is necessary for coupling with the instrumentchannel of an endoscope but it can also be advantageous for automatictesting systems in which the optical fibre bundle, with a focusingoptical head at its end, is manipulated automatically like a measuringarm on a sample matrix. Moreover, independently of endoscopic use,miniaturization of the optical head is also advantageous for increasingpositioning precision and also for minimizing mechanical inertia inautomated uses.

More particularly, the equipment according to the invention is of thetype comprising a source emitting radiation of a given wavelengthproducing a parallel illumination beam. This illumination beam is thenseparated for example by a separating plate in order to split theillumination path and the detection path. It is then deflected angularlyin two spatial directions (scanning) by an optomechanical system ofmirrors. An optical means then picks up the beam scanned angularly andinjects it into an image guide situated in the focal plane of the latterand constituted by an organized bundle of several tens of thousands offlexible optical fibres. Thus, at a given moment, one of the opticalfibres of the image guide is injected for a given angular position ofthe bundle. Over time, the optical fibres constituting the image guideare injected successively, by angular deflection of the beam by means ofthe mirrors, point-by-point for a given line, and line-by-line in orderto constitute the image. The bundle injected into the image guide (ifappropriate previously arranged in the instrument channel of anendoscope) is guided, emerges from it and is picked up by an opticalmeans allowing illumination point-by-point of the site which is to beobserved. At any moment, the spot illuminating the tissue isbackscattered and follows the reverse path of the incident beam. Thisbackscattered flux is then reinjected into the image guide, emerges fromit, reaches the scanning system, is then returned on the detectionpathway by means of the separating plate, then focussed in a filteringhole. It is then detected for example by a photomultiplier or anavalanche photodiode. The signal originating from the photodetector isthen integrated, then digitized in order to be displayed on a screen.

A device of this type is described in particular in International PatentApplication WO 00/16151.

In the case of the analysis of a biological tissue, the difficultiesthat are encountered are linked to the low ratio of useful backscatteredsignal to parasitic signal, which, in order for the image produced to beacceptable, requires a quality of illumination beam which is the bestpossible and preserved throughout the optical path, in particularregarding the quality of the wave front and the spatial distribution ofthe focal spot intensity which must be as close as possible to thediameter of a fibre core. On the side of the proximal end of the imageguide, the degradation of the illumination beam with respect to bothenergy and space is in particular due to the parasitic reflectionsoccurring at the image guide input and to optical transmission faults atthe scanning and injection systems (field deformation, wave fronterror).

In International Patent Application WO 00/16151 mentioned above, thescanning system comprises optomechanical resonating and/or galvanometricmirrors and the system for injecting into the image guide comprises afocusing lens L4 or microscope objective.

OBJECT OF THE INVENTION

The present invention has the aim of proposing an equipment with animproved quality of the illumination beam at the image guide input andas a result an image quality also improved. It also has the aim ofproposing a solution, which is low-cost, simple to implement, and whichcan be miniaturized and produced industrially.

SUMMARY OF THE INVENTION

It proposes a confocal imaging equipment in particular for an endoscopecomprising an image guide constituted by flexible optical fibres with:

on the side of the proximal end of the image guide: a source producingan illumination beam, means for angular scanning of said beam, means forinjecting the beam deflected alternately into one of the fibres of theimage guide, means for separating the illumination beam and thebackscattered signal, means for spatial filtering, means for detectingsaid signal, electronic means for controlling, analyzing and digitalprocessing of the detected signal and for display; and

on the side of the distal end of the image guide: an optical headadapted for focusing the illumination beam coming out of the illuminatedfibre.

The invention is characterized in that the means for angular scanningcomprise a resonating line mirror and a galvanometric frame mirror witha variable frequency and two afocal optical systems adapted to conjugatefirst the two mirrors then to conjugate the frame mirror and the meansfor injection into the image guide, each optical system respecting theinitial wave front quality (WFE) and having a spatial distribution ofthe focal spot intensity (PSF) equal to the diameter of a fibre core.

Thanks to these optical means, it is possible to guarantee a quality ofthe illumination beam and a homogeneous and optimal level of couplingfibre by fibre.

Each optical system can comprise either a set of standard lenses makingit possible to carry out the scanning and the injection into the imageguide coupled with custom-made additional lenses having the function ofcorrecting the residual aberrations of standard lenses, or a set ofcustom-made lenses of very good quality.

According to a particular example, an afocal optical system comprisesfour lenses, a corrective doublet of which being placed symmetricallyrelative to the image plane makes it possible to correct the curvatureof field and minimize the wave front error.

In order to further minimize the residual aberrations, the means forinjection into the image guide comprises a set of lenses for convertingthe angular scanning of the illumination beam to a translationalscanning of the image guide which comprises upstream a doublet adaptedto correct the residual curvature of field of said set of lenses.

Advantageously according to the invention, the electronic means forcontrolling, analyzing and digital processing of the detected signal anddisplay means comprise a synchronization card adapted in particular forcontrolling in a synchronized manner the movement of the line and framemirrors and adapted to know at any moment the position of theillumination beam scanned.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be better understood and other advantageswill become evident in light of the description which follows of anembodiment, which description refers to FIG. 1 in which an equipmentaccording to said embodiment is represented diagrammatically.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an equipment is proposed for producing an image of a sitesituated at a given depth in a plan P of section XY perpendicular to theoptical axis, said equipment comprising an image guide 1 constituted byseveral tens of thousands of flexible optical fibres with:

on the side of the proximal end of the image guide 1: a source 2producing an illumination beam, means for angular scanning 3 of saidbeam, means for injecting 4 the beam deflected alternately into one ofthe fibres of the image guide 1, means for separating 5 the illuminationbeam and the backscattered signal, means for spatial filtering 6, meansof detecting 7 said signal, electronic means 8 for controlling,analyzing and digital processing of the detected signal and fordisplaying; andon the side of the distal end of the image guide 1: an optical head 9adapted for focusing the illumination beam leaving the illuminated fibreof the image guide into a focussed point 10 in the plane P under thecontact zone 11 of the optical head 9.

All these means are described hereafter in detail.

The image guide 1 allows access to the subsurface analysis zone bytransporting the source 2. If it is intended, with the optical head 9,to be inserted into the instrument channel of the endoscope, it musthave dimensions which are compatible (a few millimetres in diameter inaccordance with clinical use). It is constituted by an organized bundleof flexible optical fibres surrounded by a sheath. Any guide havingenough fibres and a small inter-core spacing can be used in order toobtain a good spatial resolution. By way of example, a guide ofSumitomo® trademark can be used constituted by 30,000 fibres with a corediameter of 2.5 μm and inter-core spacing of 4 μm, or a guide ofFujikura® trademark constituted by 30,000 fibres with a core diameter of2 μm and inter-core spacing of 3.7 μm. According to the invention, thefibres are illuminated one by one by turns and in an addressed manner,using the scanning means 3 and injection means 4. The useful diameter ofthe image guide therefore corresponds to the core diameter of anilluminated fibre.

The image guide 1 is equipped at both ends with glass plates 16, 17thick enough to reject the parasitic reflections outside the filteringmeans 6 for the reflection occurring at the fibre bundle input, andoutside the illuminated optical fibre for the reflection occurring atthe image guide output. The glass plates undergo anti-reflectiontreatment in order to minimize the light reflected.

The source 2 is constituted by a 683 nm laser diode which must have avery good wave front quality, less than or equal to λ/10. According tothe invention, this diode is pulsed in order to split by synchronousdetection the useful signal from the parasitic reflection occurring atthe image guide 1 input. As a variant, a solid or gas laser can be used,but the choice of wavelength in the 600-800 nm band where absorptioninto the tissues is lower, is less extensive; moreover, the equivalentpower cost is much greater.

The means 5 for separating the illumination beam and the backscatteredsignal are constituted here by a 50/50 separating cube for adjustmentfacilities. A 50/50 separating plate can also be used.

The scanning means 3 have the function of reproducing a diode matrix ofthe same optical quality as the laser diode of the source 2 and which isto be injected fibre by fibre. This requires a combination ofnon-standard optical means allowing correction of the aberrations thatare present in the transport and source duplication system in order toilluminate the signal guide fibre by fibre. The scanning system isconstituted by two mirrors M1 and M2 and two optical systems. The mirrorM1 is a “line” mirror resonating at a frequency of 4 kHz and the mirrorM2 a galvanometric “frame” mirror with a variable frequency between 0and 300 Hz. Each optical system is constituted by four lenses,respectively L1-L4 and L5-L8, able to conjugate first the two mirrors,then to conjugate the mirror M2 and the image guide input. These opticalsystems should not have aberrations which could:

widen the spatial distribution of the focal spot intensity (PSF: PointSpread Function) after the injection means 4 and thus degrade thecoupling in the image guide 1;

propagate the flux in the sheath of the image guide 1 which woulddegrade the PSF at the end of the guide and therefore would degrade theimage resolution.

The lenses L2-L3 and L6-L7 are identical correcting doublets placedsymmetrically relative to the image plane. This allows homogenization ofthe injection into the image guide by correcting the curvature of fieldand by minimizing the wave front error due to the use of off-axis afocalsystems (L1-L4 and L5-L8).

The injection means 4: These must have the minimum number of aberrationsand should not degrade the quality of the wave front in order to producea focal spot close to the diffraction limit in order thus to produce anoptimal coupling with the addressed fibre (a PSF equal to the diameterof a fibre core). They comprise a custom-made doublet L9 and a standardtriplet L10. The doublet L9 allows correction of the residualaberrations of the triplet L10, namely the curvature of field.

The means for spatial filtering 6 comprise a lens L11 and a filteringhole T making it possible to select only the illumination fibre and notthe adjacent fibres which can generate a parasitic signal. The size ofthe filtering hole is such that it corresponds to the diameter of afibre core, taking into account the magnification of the optical systembetween the fibre bundle input and the filtering hole.

The optical head 9 comprises several optical means allowing convergenceof the beam emerging from the illuminated optical fibre and two glassplates, one being described above at the image guide output and theother a window adapted for coming into contact with the site andproducing an index adaptation. The optical means have the followingcharacteristics:

allowing analysis of the tissue at a depth of several tens to severalhundreds of microns;

minimizing the aberrations in order to transcribe the PSF at the imageguide output on the tissue without magnifying the latter or deformingit;

optimizing the return coupling level in the image guide by optimizingthe wave front quality;

if appropriate, dimensions compatible with those of the instrumentchannel of an endoscope.

The optical means comprise for example a lens system forming acustom-made objective.

The detection means 7 comprise an avalanche photodiode as signaldetector which receives the signal continuously, the parasitic signaloriginating from the two ends of the signal guide being carried backwith the same order of magnitude as the useful signal in order not tosaturate the detector. The suppression of the parasitic reflectionresidue at the image guide input is then carried out by digital timefiltering.

The electronic means 8 for controlling, analyzing and digital processingof the detected signal and for displaying comprise the following cards:

a modulation card 20 of the laser source. This card allows modulation ofthe source at a relatively high frequency (of the order of 100 MHz) inorder to produce pulses (10 ns≦τ≦100 ns) at regular intervals (cycleratio of the order of 4).

a synchronization card 21 which has the functions:

of controlling in a synchronized manner the scanning, i.e. the movementof the line mirror M1 and frame mirror M2;

of knowing at any moment the position of the laser spot thus scanned;

of synchronizing the emission of the laser source pulses beforedetection;

of controlling all the other cards via a microcontroller which canitself be controlled;

a detector card 22 which comprises an analogue circuit which inparticular carries out an impedance adaptation and integration, adigital-analogue converter and a programmable logic component (forexample an FGPA circuit) which formats the signal;

a digital acquisition card 23 which makes it possible to process avariable-frequency digital data stream and to display it on a screen 24;

a graphics card 25.

The image processing is carried out as follows. The raw information fromthe detector card is formatted and processed so that it can bevisualized then interpreted. The process of acquisition of the imagesvia the image guide constituted by several tens of thousands of opticalfibres and by scanning of the latter leads to specificities in the imageand appropriate processing.

Two processing groups are provided:

-   1. The first group is constituted by signal processing processes    aimed at calibrating the signal received. It is thus possible to    eliminate laser/guide coupling faults inherent of the acquisition    process, as well as faults due to certain system noises. The    calibration can take different forms depending on the scanning    control precision, and its stability over time. These processing    processes are essentially mono-dimensional.-   2. The second group allows improvement of the interpretation by    integrating image processing processes (2D and 2D+time) specific to    the opto-mechanical process. These processing processes consist of    an image restoration process, followed by a rapid alignment process    allowing elimination of the small movements. These processing    processes are rapid compared with the time taken for acquisition.    These algorithms are entirely automatic and are adapted to the    nature of the image.

It goes without saying that embodiment variants are possible inparticular as regards the line mirror M1 which can resonate at anotherfrequency, for example 8 kHz, the afocal optical systems which can beentirely custom-made or also could comprise other sets of adaptedcorrective lenses.

1. Confocal imaging equipment comprising an image guide (1) constitutedby flexible optical fibres with: on the side of the proximal end of theimage guide (1): a source (2) producing an illumination beam, means forangular scanning (3) of said beam, means for injecting (4) the beamdeflected alternately into one of the fibres of the image guide (1),means for separating (5) the illumination beam and a backscatteredsignal, means for spatial filtering (6), means for detecting (7) saidbackscattered signal, electronic means (8) for controlling, analyzingand digital processing of the detected said backscattered signal anddisplay; and on the side of the distal end of the image guide (1): anoptical head (9) adapted for focusing the illumination beam leaving theilluminated fibre, characterized in that the means for angular scanning(3) comprise a resonating line mirror (M1) and a galvanometric framemirror (M2) with a variable frequency and two afocal optical systemsadapted for conjugating first the two mirrors (M1, M2) then forconjugating the frame mirror (M2) and the injection means (4) in theimage guide, each optical system respecting the initial quality of thewave front (WFE) and having a spatial distribution of the focal spotintensity (PSF) equal to the diameter of a fibre core; and in that anafocal optical system comprises standard lenses and corrective lensesadapted for correcting the residual aberrations of said standard lenses.2. Equipment according to claim 1, characterized in that the afocaloptical systems comprise eight lenses (L1-L4; L5-L8) a correctivedoublet (L2, L3; L6, L7) of which is placed symmetrically relative tothe image plane allowing correction of the curvature of field andminimization of the wave front error.
 3. Equipment according to claim 1,characterized in that the injection means (4) comprise a set of lenses(L10) adapted for converting the angular scanning to translationalscanning of the image guide and upstream a doublet (L9) adapted forcorrecting the residual curvature of field of said set of lenses (L10).4. Equipment according to claim 3, characterized in that said set oflenses (L10) is a triplet.
 5. Equipment according to claim 1,characterized in that it comprises a glass plate (16) arranged at animage guide input intended to reject the parasitic reflections outsidethe filtering means (6).
 6. Equipment according to claim 1,characterized in that it comprises a glass plate (17) arranged at animage guide output intended to reject parasitic reflections outside theilluminated optical fibre.
 7. Equipment according to claim 1,characterized in that the line mirror (M1) is a mirror resonating at afrequency of 4 kHz.
 8. Equipment according to claim 1, characterized inthat the frame mirror (M2) has a variable frequency between 0 and 300Hz.
 9. Equipment according to claim 1, characterized in that theelectronic means (8) for controlling, analyzing and digital processingof the detected signal and display comprise a synchronization card (21)adapted in particular for controlling in a synchronized manner themovement of the line mirror (M1) and frame mirror (M2) and adapted toknow at any moment the position of the scanned illumination beam.